Numerous methods have been used to prevent metals, particularly structural metals, from corroding in the atmosphere. The most commonly used methods include (1) barrier coating; and (2) cathodic protection, i.e. providing a "sacrificial" metal coating which is anodic to the metal substrate so that the "sacrificial" metal coating will be corroded before the metal substrate is attacked. Zinc has been widely used for this purpose, and may be applied in the form of a zinc rich paint on the metal substrate or by galvanization. Galvanization with zinc metal is the most commonly used method to improve corrosion resistance of ferrous metals and steels. Processes for galvanizing metal substrates include hot-dip, hot-spray and electrodeposition.
However, the zinc rich paints tend to contain non-conductive binders which coat the zinc particles and prevent the "sacrificial" galvanic reaction from proceeding. This undercuts the effectiveness of zinc rich paints as a protective coating. Galvanization by a hot-dip or hot-spray process consumes large quantities of energy and is very costly. Moreover, galvanization results in a brittle, macro-crystalline zinc coating which is difficult to form and will not accept paint except after the zinc surface has been treated by chromate conversion or phosphating.
It is known that zinc may be electroplated from acid solution at pH of about 3 to 4.5. See Frederick A. Lowenheim ed., Modern Electroplating, 3rd Ed., John Wiley & Sons, New York (1974), pp. 442-460. However, these acid zinc plating solutions have not been used extensively commercially to product zinc deposits having high ductility and good adhesion to difficult to plate metals. Without being bound by theory, it is believed that this is because these previous deposits are interfered with by the presence of detrimental inclusions in the crystal lattice, thus decreasing their ductility and adhesion characteristics. The zinc electrodeposited according to the present invention does not have these detrimental inclusions in the crystal lattice. Based on experiments none of the known acid zinc electroplating processes provide ductile deposits which can be bent or deformed and still provide sufficient corrosion resistance even when chromated.
It is also known that zinc coatings can be further treated. These treatment processes include phosphating and chromate conversion coating. Chromate conversion coating significantly improves the corrosion resistance of metal substrates which are galvanized or electroplated with zinc. Phosphating is used to improve adhesion of paints to galvanized surfaces. However, the chromate conversion and phosphating processes also result in brittle coatings which cannot be formed except with difficulty.
Metals are also surface treated to provide galling and wear resistance and lubricity. For example, metals have been coated with cadmium to provide the desired properties. However, cadmium is highly toxic and there are stringent federal and local regulations controlling its use. This increases the cost and limits its application.
Other methods of providing galling and wear resistance and improved lubricity include oxalate conversion coating, coating with fluorocarbon polymers and coating with electroless copper, nickel or hard chromium.
However, oxalate conversion does not provide corrosion resistance. The usable temperature range of fluorocarbon polymers is very limited and there tends to be excessive flow under stress. Therefore, it is not suitable for applications where the metal substrate is to be subjected to high temperatures and stress. Copper coatings induce corrosion of ferrous metal substrates. Electroless nickel and hard chromium coatings tend to break down under very high stress loads. Moreover, both are very hard coatings which provide wear resistance but poor galling resistance and lubricity. These disadvantages reduce the desirability of using these methods to provide simultaneously galling and wear resistance and improved lubricity.
Metal substrates may also be "siliconized" or implanted with phosphorus to improve their wear resistance. However, these processes are difficult to control, expensive and are impractical.
Another serious problem recently encountered is stress corrosion cracking of high strength alloys. These high strength alloys are used in satellites, space vehicles, airplanes, cars, bridges and nuclear reactors and are subjected to highly stressful environments. Stress failures have been the major causes of airplane and auto crashes, flaws in bridges and nuclear reactors. No viable solution was available to solve the stress cracking of high strength alloys until recently. A method of electroplating a zinc/silicon/phosphorus coating on high strength alloys to provide improved stress corrosion cracking resistance was described in co-pending application U.S. Ser. No. 641,557 (now U.S. Pat. No. 4,533,606) also assigned to the same assignee as this application.
Another area where the present invention has application is in the plating of difficult to coat metal substrates, such as aluminum, titanium and stainless steel. These metal substrates are difficult to coat because of the presence of a film of metal oxide on the surface. The metal oxide film can be removed by immersion in acidic or alkaline solutions. However, the oxide film reforms immediately when the metal substrate is removed from the de-oxidizing solution.
Phosphating with zinc oxide in phosphoric acid has also been used to improve adhesion of paints of difficult to coat metals. See U.S. Pat. No. 2,743,205. However, phosphating makes the surface very brittle, so that the treated metal article cannot be formed without losing corrosion resistance. Moreover, large quantities of sludge are produced in the process and must be properly disposed. Otherwise, there will be severe damage to the environment. Although methods to improve adhesion to difficult to coat metals are available, these generally involve a multiplicity of processing steps and high production costs.
It is desirable, therefore, to provide a simple method for improved adhesion on these metals.
In U.S. Ser. No. 641,557, owned by the same assignee, a composition and process for electroplating an adhesive zinc/silicon/phosphorus coating highly resistant to corrosion, stress corrosion cracking, wear and galling was described. However, the process is quite complicated, difficult, and time consuming.
It is, therefore, an objective of the present invention to provide an efficient and simple method to produce a ductile, adhesive zinc coating that has all of the desirable properties: resistance against corrosion, stress corrosion cracking, wear and galling and is formable.
It is a further objective of the present invention to provide an efficient and simple method to produce a ductile, adhesive zinc coating that has all of the desirable properties; resistance against corrosion, stress corrosion cracking, wear and galling and is formable, wherein the electroplating solution may be operated at room temperature, e.g., 15.degree. C. to 35.degree. C.
It is a further objective of the present invention to provide an efficient and simple method to produce a ductile, adhesive zinc coating that has all of the desirable properties; resistance against corrosion, stress corrosion cracking, wear and galling; and is formable to provide a coating surface which is dull and non-lustrous and is an excellent base for barrier coatings including paint, adhesives and other electrocoatings.
It is a further object of the present invention to provide a simple electroplating process to deposit an adhesive coating on difficult to coat metals to provide a substrate for accepting a desired coating.